Method and apparatus for measuring cardiac output via an extracorporeal cardiopulmonary support circuit

ABSTRACT

A method and apparatus for determining cardiac output in conjunction with flow through an extracorporeal circuit, wherein flow through an arterial line of the extracorporeal circuit is temporarily reversed and an indicator is passed through the cardiopulmonary circuit. A dilution curve is measured in the arterial line of the extracorporeal circuit during the reversed flow, and cardiac output is determined corresponding to the measured dilution curve.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A “SEQUENCE LISTING”

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to determining cardiac output of apatient, and more particularly, to determining the cardiac output of apatient through an extracorporeal cardiopulmonary support circuit,wherein flow through at least a portion of the extracorporealcardiopulmonary support circuit is reversed and a measurement at thereversed flow is taken from which the cardiac output is determined.

2. Description of Related Art

Respiratory failure requiring pulmonary support affects in excess of300,000 people in the United States per year. Approximately one-half ofthese patients suffer from adult respiratory distress syndrome (ARDS).Adult respiratory distress syndrome is an acute inflammatory lungdisease with a mortality rate of 50%. This disease is characterized byincreased capillary permeability resulting from the development ofinterstitial edema and alveolar flooding. For the vast majority ofpatients with ARDS, there is no specific treatment, or supportivetherapy. Supportive therapy for ARDS focuses on mechanical ventilation.An alternative life support modality, such as extracorporealoxygenation, can be a therapeutic option for acute respiratory failurein both infants and adults.

In addition, extracorporeal circulation (perfusion) is used for the mostpart in cardiac bypass surgery. In a total bypass, all the systemicvenous return blood of the patient is diverted from entering the rightside of the heart and into an extracorporeal circuit. In suchapplication, the extracorporeal circuit includes a heart-lung machinethat comprises a pumping function and an oxygenation function,completely taking over cardiopulmonary function for the patient,returning oxygenated blood to the aorta, downstream of thecardiopulmonary circuit. In a partial bypass only a portion of the bloodis diverted to the extracorporeal circuit, the remaining flow passing tothe heart, the lungs and from the lungs through heart to the systemiccirculation.

A use of extracorporeal circulation as “extracorporeal life support” caninclude “extracorporeal membrane oxygenation” known by the respectiveacronyms of “ECLS” or “ECMO”, for simplicity herein called ECMO. Asopposed to the more conventional extracorporeal circulation andsubstitution or assist of the cardiac function, ECMO connotes theapplication of such support to supply oxygenation where the native lungsmay be compromised. This is especially useful for neonates, includingpremature birth babies, whose life is threatened because their immaturelungs cannot provide adequate gas exchange. Another use is resuscitateddrowning victims whose lungs are damaged and unable to supply adequateoxygenation without restorative healing. The extracorporeal circulationprovides oxygenated blood to the lungs under the impetus of thepatient's native heart and gives time to allow healing of the lungs tooccur until the lungs can take over oxygenation. In excess of 1,000 ECMOprocedures are conducted annually in the United States.

Another use of extracorporeal circulation is to provide heart supportwithout supplementary oxygenation. For example, part of the blood flowbypasses the heart and instead passes through the extracorporealcircuit, thereby reducing a portion of the load on the heart.

While the applications and successes of extracorporeal circulation havebeen increasing, the need remains for limiting the duration of theextracorporeal circulation to a substantially as needed basis. The needexists for determining the as needed basis in terms of measuring patientperformance during the extracorporeal circulation. A need exists fordetermining cardiac output during extracorporeal circulation. The needexist for determining cardiac output without requiring furtherintervention or cessation of treatment.

BRIEF SUMMARY OF THE INVENTION

One configuration provides a method and apparatus for determiningcardiac output of a patient on extracorporeal circulation including, butnot limited to extracorporeal life support. By monitoring the cardiacoutput, the extracorporeal circulation can be controlled to correspondto the capacity of the patient so as to minimize excessiveextracorporeal circulation time. In addition, selected configurationscan provide real time assessment of heart performance, and particularlyas in response to a substantially contemporaneous or prior treatment.

The present method provides for measuring a patient cardiac output withan extracorporeal cardiopulmonary support device withdrawing blood froma venous portion of a patent vascular system through an extracorporealvenous line and delivering blood to an arterial portion of the patientvascular system through an arterial extracorporeal line, thentemporarily reversing flow in the extracorporeal venous line and theextracorporeal arterial line for withdrawing the blood and a portion ofthe dilution indicator, measuring a dilution curve in the extracorporealsystem and determining a cardiac output corresponding to the measureddilution curve. Preferably, the dilution curve is measured in thearterial line of the extracorporeal circuit.

The apparatus for determining the cardiac output includes means forreversing the flow in at least the arterial line of the extracorporealcircuit, means for introducing a dilution indicator to pass through thecardiopulmonary circuit, a sensor for measuring a dilution curve of thedilution indicator in the extracorporeal circuit, and preferably in thearterial line, and a controller for determining the cardiac outputcorresponding to the measured dilution curve.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 is a schematic of the circulatory system showing thecardiopulmonary and systemic circuit with a connected extracorporealcircuit in a normal, or forward extracorporeal flow configuration.

FIG. 2 is the schematic of FIG. 1 showing a reversed extracorporeal flowconfiguration, such as by extracorporeal pump reversal.

FIG. 3 is a schematic of an extracorpored circuit with a reversedextracorporeal flow configuration, such as by extracorporeal linereversal.

FIG. 4 is a schematic of an extracorporeal circuit with a reversedextracorporeal flow configuration, such as by extracorporeal bridgecontrol.

FIG. 5 is a schematic of an extracorporeal circuit with bridge tubing ina forward extracorporeal flow configuration.

FIG. 6 is a schematic of the extracorporeal circuit of FIG. 5, withselective flow in the bridge tubing to provide a reversed flow of theextracorporeal circuit.

FIG. 7 is a graphical representation of a dilution curve measured at twolocations in the extracorporeal circuit.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an extracorporeal circuit 100 is shown connected toa circulation system 20.

The circulation system 20 is a human (or animal) circulatory systemincluding blood, a vascular system, and a heart. For purposes of thisdescription, the circulation system 20 is represented as acardiopulmonary system 30 and a systemic system 40 connecting thecardiopulmonary system 30 to the tissues of the body. Specifically, thesystemic system 40 passes the blood though the vascular system(arteries, veins, and capillaries 42) throughout the body. The portalcirculation is taken as part of the systemic system 40.

The cardiopulmonary system 30 includes the right heart, the lungs andthe left heart, as well as the vascular structure connecting the rightheart to the lungs, the lungs to the left heart and some portion of theaorta and large veins located between the extracorporeal circuit and theright and left heart. That is, in theory the cardiopulmonary system 30would include only the right heart, the lungs, the left heart and thevascular structure directly connecting the right heart to the lungs andthe lungs to the left heart. However, in practice it is sometimesimpracticable to operably connect the extracorporeal circuit 100immediately adjacent the large vein at the right heart, or immediatelyadjacent the aorta at the left heart. Therefore, the cardiopulmonarysystem 30 often includes a limited length of the vein entering the rightheart and the aorta exiting the left heart. For example, theextracorporeal circuit 100 can be connected to a femoral artery andfemoral vein, thereby effectively extending the cardiopulmonary system30 to such femoral artery or vein.

For cardiopulmonary and vascular systems, the term “upstream” of a givenposition refers to a direction against the flow of blood, and the term“downstream” of a given position is the direction of blood flow awayfrom the given position. The “arterial” side or portion is that part inwhich oxygenated blood flows from the heart to the capillaries. The“venous” side or portion is that part in which blood flows from thecapillaries to the heart and lungs (the cardiopulmonary system 30).

The basic components of the extracorporeal circuit 100 for aconventional heart-lung machine include a venous line 110, a venousreservoir (not shown), an oxygenator 120 and heat exchanger (not shown),a pump 130, an arterial line filter (not shown), an arterial line 140, adilution sensor 146 in the arterial line and a controller 160

Generally, the extracorporeal circuit 100 withdraws blood from thevenous portion of the circulation system 20 (or cardiopulmonary system30), and returns the blood to the arterial portion of the circulationsystem. The withdrawn blood can be treated while it is withdrawn, or thewithdrawn blood can be merely returned to the arterial portion of thecirculation system 20. The blood treatment, if applied, can be any of avariety of treatments including, but not limited to, oxygenation (andcarbon dioxide withdrawal).

The venous line 110 extends from the venous portion of the circulationsystem 20, and preferably from a venous portion of the cardiopulmonarysystem 30. The venous line 110 typically includes a venous cannula 112providing the fluid connection to the circulation system 20.

The venous line 110 can also include or provide a site 114 forintroduction of the dilution indicator. In a preferred configuration,the site 114 for introducing the dilution indicator is proximal to theinterface of the venous line 110 and the circulation system 20. Inselected configurations, the introduction site 114 can be integratedinto the venous cannula 112.

If the venous dilution senor 116 is used, then the injection site 114should be distal to the sensor so when flow is reversed, the injectedindicator first passes the sensor prior entering venous system throughcannula 112.

It is also contemplated, that a component of the extracorporeal circuit100 can be controlled to create or induce an indicator within the flowin the extracorporeal circuit. For example, a filtration or treatmentrate can be sufficiently changed to create an effective indicator in theextracorporeal circuit 100 which then travels through thecardiopulmonary system 30.

In addition, the venous line 110 can include a dilution sensor 116. Thedilution sensor 116 (as well as sensor 146) can be any of a variety ofsensors, and can cooperate with the particular indicator. The sensor 116(as well as sensor 146) can measure different blood properties: such asbut not limited to temperature, electrical impedance, opticalproperties, density, ultrasound velocity, concentration of glucose andother blood substances (any physical or chemical blood properties).Preferably, the sensor 116 is close to the venous point of cannulation112, (the interface of the venous line 110 and the circulation system20).

The arterial line 140 connects the extracorporeal circuit 100 to anarterial portion of the circulation system 20 and preferably to anarterial portion of the cardiopulmonary system 30. The arterial line 140usually connects to the ascending aorta. However, the arterial line 140can be placed downstream in the arterial portion of the vascular systemfor example into femoral artery, or carotid artery, where the vessel issufficiently large to accommodate the necessary flow rate. The arterialline 140 typically includes an arterial cannula 142 providing the fluidconnection to the circulation system 20.

The arterial line 140 also includes the dilution sensor 146. The sensor146 can be any of a variety of sensors, as set forth in the descriptionof the sensor 116, and is typically selected to cooperate with theanticipated indicator. Preferably, the sensor 146 is proximal (close to)to the point of arterial cannulation, (the interface of the arterialline 140 and the circulation system 20).

However, it is understood the sensor 146 can be located anywhere in theextracorporeal circuit 100 or even outside of the extracorporealcircuit. That is, the sensor 146 can be remotely located and measure inthe extracorporeal circuit 100, and preferably in the arterial line 140,the changes produced in the blood from the indicator introduction orvalues related to the indicator introduction which can be transmitted ortransferred by means of diffusion, electromagnetic or thermo fields orby other means to the location of the sensor.

Current oxygenators 120 are broadly classified into bubble type andoxygenators and membrane type oxygenators. The membrane type oxygenatorsfall under the laminate type, the coil type, and the hollow fiber type.Membrane type oxygenators offer advantages over the bubble typeoxygenators as the membrane type oxygenators typically cause less blooddamage, such as hemolysis, protein denaturation, and blood coagulationas compared with the bubble type oxygenators. Although the preferredconfiguration is set forth in terms of a membrane type oxygenator, it isunderstood any type of oxygenator can be employed or no oxygenator canbe used in the extracorporeal circuit.

The pump 130 can be any of a variety of pumps types, including but notlimited to a roller (or impeller) pump. The pump 130 induces a bloodflow through the extracorporeal circuit 100. At least one of the pump130 and the controller 160 typically include control of the rpm of thepump and the flow rate of the blood through the pump, respectively. Thepump 130 can be at any of a variety of locations in the extracorporealcircuit 100, and is not limited to the position shown in the Figures.

The controller 160 is typically connectable to the oxygenator 120, thepump 130 and the sensor(s) 116, 146. The controller 160 can be a standalone device such as a personal computer, a dedicated device or embeddedin one of the components, such as the pump 130 or the oxygenator 120.Although the controller 160 is shown as connected to the sensors 116 and146, the pump 130 and the oxygenator 120, it is understood thecontroller can be connected to only the sensors, the sensors and thepump, or any combination of the sensors, pump and oxygenator.

The normal or forward blood flow through the extracorporeal circuit 100includes withdrawing blood through the venous line 110 from the venousside circulation system 20 (and particularly the cardiopulmonary circuit30), passing the withdrawn blood through the extracorporeal circuit (tooptionally treat such as oxygenate, or merely circulate the withdrawnblood), and introducing the withdrawn (or treated) blood through thearterial line 140 into the arterial side of the circulation system (andparticularly the arterial portion of the cardiopulmonary circuit). Thepump 130 thereby normally induces a blood flow through theextracorporeal circuit 100 from the venous line 110 to the arterial line140.

Thus, the forward flow through the extracorporeal circuit 100 isparallel to the flow from the venous side of the circulation system 20(through the cardiopulmonary system 30, the right heart, the lungs, aleft heart) and passing to the arterial portion of the systemic system40.

While cardiopulmonary support offers life-saving and life prolongingtreatment, the intrusive nature of the cardiopulmonary support carriessignificant risks and potential complications. Each additional hour ofunnecessary cardiopulmonary support increases the probability ofnegative complications as well as increasing the already substantialcosts of the treatment. Therefore, it is desirable to limit the durationof cardiopulmonary support as required by the individual patient. One ofthe main criteria for decreasing or terminating cardiopulmonary supportis an adequate increase in the heart flow—cardiac output. Typically,cardiopulmonary support can be decreased as the normal heart capacity isrestored.

The heart capacity (flow) is typically measured by cardiac output CO.Cardiac output CO is the amount of blood pumped out by the leftventricles in a given period of time (typically a 1 minute interval).

Referring to FIG. 1, the total blood flow (TBF) passing to the systemicsystem 40 is the sum of the cardiac output CO (blood flowing from thecardiopulmonary system 30) and the blood flow Q from the extracorporealcircuit 100.TBF=CO+Q  (Eq. 1)

To apply the present dilution technique to measure cardiac output COduring circulation in the extracorporeal circuit 100, the blood flow isreversed in at least the arterial line 140, and the venous line 110 ifthere is no buffer in the extracorporeal circuit 100. Thus, blood iswithdrawn from the arterial side of the circulation system 20, passedthrough the extracorporeal circuit 100 and delivered to the venous sideof the circulation system 20. That is, the flow in the extracorporealcircuit 100 is reversed relative to the circulation system 20. Duringthe reversed flow in the extracorporeal circuit 100, the dilutionindicator injected into the venous line (of extracorporeal circuit 100or intravascular system, such as circulation system 20) will passthrough any incorporated portion of the venous portion of thecirculation system 20, the cardiopulmonary circuit 30 (the right heart,the lungs, the left heart) and will be sampled (dilution curve recordedor measured) in the of the extracorporeal circuit, and preferably inarterial line 140.

The flow can be reversed by a number of configurations. Referring toFIG. 2, the direction of the pump 130 can be temporarily reversed (forexample, on the order of 2-5 minutes). In this case, injection of theindicator can be made anywhere into the extracorporeal circuit 100,though preferably at the site 114 in the venous line 110. The indicatorwill then pass through the cardiopulmonary system 30 and a portion ofthe indicator then passes into the arterial line 140 of theextracorporeal circuit 100. The resulting dilution curve can be measuredanywhere in the extracorporeal circuit 100, and preferably in thearterial line 140. The resulting dilution curve is recorded, or measuredand from which the cardiac output CO is calculated.

However, components of the extracorporeal circuit 100, such as (bubbletraps, oxygenators) are typically designed to permit only singledirection (normal or forward) flow. Therefore, for this configuration tobe implemented, the components of the extracorporeal circuit 100 must becompatible for bidirectional flow.

Alternatively, as seen in FIG. 3, the lines between the pump 130 and thecirculation system 20 (patient) can be reversed. This line reversal canbe achieved by, for example, using a special line reversal device, lineswitching or with simple re-clamping of tubing in the extracorporealcircuit 100 (an example of the line clamping is seen in FIGS. 5 and 6).An advantage to this configuration of the flow reversal lies in aconstant forward flow through the pump 130 and relevant extracorporealcomponents. Thus, only in the local region of the interface between theextracorporeal circuit 100 and the circulation system 20 (patient) isthe flow reversed so that indicator if introduced into theextracorporeal circuit 100 passes into the venous portion of thecardiopulmonary system 30, through the cardiopulmonary system and aportion is withdrawn through the arterial line 140 of the extracorporealcircuit. Again, the dilution curves can be measured anywhere within theextracorporeal circuit 100, but preferably along the arterial line 140.

In the extracorporeal circuit 100 of some cardiopulmonary supportsystems, such as ECMO, there is a bridge (bypass) tubing 170 between thewithdraw and return lines, as seen in FIG. 4). The bridge tubing 170 isusually clamped to preclude flow therethrough. However, the bridgetubing 170 is periodically opened to insure that blood is not clotted.The bridge tubing 170 is established for emergency situations, forexample to prevent flow from the ECMO if bubbles or clots appear orthere is a need to substitute extracorporeal lines. The bridge tubing170 can be used to measure the cardiac output CO. Specifically, the pump130 can be stopped, clamped off by clamps 150 (shown in FIG. 4) or runthrough a parallel circuit (not shown). When the pump 130 is stopped(clamped or run through the parallel circuit), the blood will flow inthe bridge tubing 170 by patient pressure gradient from the artery tovein. This flow can allow performing cardiac output measurements, asseen in FIG. 4. However, this configuration of flow reversal is subjectto the limitation that blood flow through the bridge tubing 170 dependson heart performance (and hence pressure from the systemic system) andhydrodynamics resistance (length of the bridge, the diameter of thetubing as well as the size of the cannula). These factors can reduce thestability of the blood flow rate through the bridge tubing 170. It isalso understood that an additional pump (not shown) can be included inthis configuration to assist in providing an acceptable flow.

Referring to FIG. 5, the extracorporeal circuit 100 can include thebridge tubing 170 to define a flow reversing structure in cooperationwith clamps 150. That is, the extracorporeal circuit 100 can be clamped,as seen in FIG. 5, to provide normal, forward flow through the circuit(the pump 130 and oxygenator 120), with no flow through the bridgetubing. Although four clamps 150 are shown it is understood fewer, ormore clamps can be used are desired to maintain the forward flow.

Referring to FIG. 6, wherein the clamps are removed (or selectivelyopened) on the bridge tubing 170 and closed on the shown portions of thevenous line and arterial line, flow is diverted through the bridgetubing to effect a reversed flow through the venous cannula 112 and thearterial cannula 142.

For each of the flow reversal configurations, it is understood thereversed flow is selected to reduce or minimize stress to the heart (andcardiopulmonary system 30).

After flow in at least the parts of the venous and the arterial lines110, 140 of the extracorporeal circuit 100 is reversed, the indicator isintroduced so as to pass from the venous portion of the circulationsystem 20 to pass through the cardiopulmonary system 30 and be withdrawninto the arterial line 140 of the extracorporeal circuit 100 to bemeasured (recorded) by the sensor 146 in (or associated with) theextracorporeal circuit. The indicator can be introduced via any place ofthe extracorporeal circuit 100, and preferably the venous line 100 ofthe extracorporeal circuit, as well as intravenously into the venousportion of the circulation system 20.

In one configuration of the invention, a single dilution indicatorsensor 146 is employed in the arterial line. As set forth in thedescription of the extracorporeal circuit 100, the preferred location ofthe single sensor 146 is close to the place of arterial cannulation,seen at 142 in FIG. 2, and the preferred location for indicatorintroduction is near the site of venous cannulation seen at 112 in FIG.2. In this construction, the indicator will be less dispersed in thelines of extracorporeal circuit 100.

It is also understood that location of dilution sensor 146 in thearterial line 140 is close to the arterial cannula 142 is beneficial asthe measured or recorded dilution curves will be less disturbed afterleaving cardiopulmonary system 30, than if recorded in the venous line110 after passing through the pump 130 or other components ofextracorporeal system 100.

The resulting dilution curve is measured or recorded by sensor, such asin the extracorporeal system 100.

The cardiac output CO will be given by: $\begin{matrix}{{CO} = \frac{Vi}{s}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$

Where Vi is the volume of the introduced indicator and S is the area ofthe dilution curve (concentration of indicator) measured in theextracorporeal circuit 100 from the blood (and indicator) flowing fromthe arterial circulation system (cardiopulmonary circuit 30). Theintroduction of the indicator can be a relatively quick (short duration)injection, a timed or measured injection, or a continuous injection.

It is understood that different formula can be used to determine thecardiac output, depending upon the specific indicator, the way theindicator is introduced, as is described in literature and textbooks.

It is also contemplated that the indicator can be introduced in thearterial line 140 just upstream of the sensor 146 (in FIGS. 2-4), suchthat a first dilution curve S1 is measured (recorded) prior to theindicator passing the remainder of the extracorporeal circuit 100 andsubsequently entering the cardiopulmonary system 30, then passing intothe arterial line to be measured again to provide a second dilutioncurve S. In this configuration, the cardiac output CO is given by:$\begin{matrix}{{CO} = {Q \cdot \left( \frac{S1}{S} \right)}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

If the sensor measures the pump flow Q, or it is known, then the cardiacoutput CO can be calculated by equation 3, were S1 and S are areas underconcentration curves or values that are proportional (related) to suchcurves. It is understood that different formula can be used to determinethe cardiac output, depending upon the specific indicator and the waythe indicator is introduced.

In the two sensor configuration, as shown in FIG. 2, (the second sensor116 being disposed in the venous line 110 downstream of the indicatorinjection site), for two matched sensors, the cardiac output CO is givenby: $\begin{matrix}{{CO} = {Q \cdot \left( \frac{S1}{S} \right)}} & \left( {{{Eq}.\quad 3}a} \right)\end{matrix}$

Where S1 is the area under the dilution concentration curve measuredbefore the indicator enters the circulation system 20 by the sensor 116on the venous line 110, or value related to the dilution concentration.

It is further contemplated that analogous sensor and introduction sitescan be employed for the configuration of FIG. 3.

As a representative example of the configuration of FIG. 2, the sensorsare ultrasound dilution sensors, and wherein sensor 116 or 146 alsomeasures blood flow in the extracorporeal circuit, and particularly thevenous line. Injection of a 10 ml of saline indicator produces firstdilution curve (1) from the sensor 116, seen in FIG. 7, and after theindicator has traveling through the cardiopulmonary system 30, thesensor 146 produces second dilution curve (2). From these measurements,the cardiac output is calculated via Equation 3a. In FIG. 7, the scaleof the first curve is different than the scale of the second curve.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, the presentinvention is intended to embrace all such alternatives, modifications,and variations as fall within the spirit and broad scope of the appendedclaims.

1. A method of measuring a patient cardiac output with an extracorporealcardiopulmonary support device withdrawing blood from a venous portionof a patent vascular system through an extracorporeal venous line anddelivering blood to an arterial portion of the patient vascular systemthrough an extracorporeal arterial line, the method comprising: (a)introducing a dilution indicator into the patient vascular system; (b)withdrawing blood including a portion of the dilution indicator from thepatient arterial vascular system through the extracorporeal arterialline; and (c) determining a cardiac output corresponding to thewithdrawn blood and dilution indicator.
 2. The method of claim 1,wherein introducing the dilution indicator into the patient vascularsystem includes passing the indicator through the extracorporeal venousline.
 3. The method of claim 1, further comprising temporarily reversingflow in the extracorporeal venous line and the extracorporeal arterialline for withdrawing the blood and a portion of the dilution indicator.4. An apparatus for determining cardiac output in conjunction with anextracorporeal cardiopulmonary support circuit having an extracorporealvenous line withdrawing blood from a venous portion of a patientvascular system, an extracorporeal arterial line introducing blood intoa patient arterial vascular system and a pump for generating a bloodflow through the extracorporeal cardiopulmonary support circuit, theapparatus comprising: (a) means for reversing flow through at least theextracorporeal arterial line; (b) a sensor for measuring passage of anindicator in the arterial line during the reversed flow; and (c) acontroller in communication with the sensor, the controller selected todetermine the cardiac output in response to the sensed indicator througharterial line.
 5. The method of claim 4, further comprising means forpassing the indicator through a cardiopulmonary circuit prior to sensingin the arterial line.
 6. A method of determining cardiac output, themethod comprising: (a) introducing a dilution indicator into a venousportion of the central blood volume; (b) withdrawing blood and a portionof the dilution indicator from an arterial portion of the central bloodvolume into an extracorporeal circuit; and (c) determining a cardiacoutput corresponding to the withdrawn portion of the dilution indicator.7. A method of determining cardiac output, the method comprising: (a)inducing a reversed flow through an extracorporeal circuit from anarterial portion of the vascular system to a venous portion of thevascular system; (b) passing an indicator through a patientcardiopulmonary circuit; (c) withdrawing, into the extracorporealcircuit, a portion of the indicator passing through the cardiopulmonarycircuit; (d) measuring a dilution curve of the indicator in theextracorporeal circuit; and (e) determining a cardiac outputcorresponding to the measured dilution curve.
 8. The method of claim 7,further comprising introducing the indicator into the extracorporealcircuit.
 9. The method of claim 7, further comprising introducing aforward flow in the extracorporeal circuit from the venous portion ofthe vascular system to an arterial portion of the vascular system, priorto inducing the reversed flow in the extracorporeal circuit.